1. Technical Field
The present invention relates generally to an improved data processing system and, in particular, to a method and system for improving data throughput within a data processing system. Specifically, the present invention relates to a method and system for improving performance of input/output processing and bus access regulation.
2. Description of Related Art
Traditionally, symmetric multiprocessors are designed around a common system bus on which all processors and other devices such as memory and I/O are connected by merely making physical contacts to the wires carrying bus signals. This common bus is the pathway for transferring commands and data between devices and also for achieving coherence among the system""s cache and memory. A single-common-bus design remains a popular choice for multiprocessor connectivity because of the simplicity of system organization.
This organization also simplifies the task of achieving coherence among the system""s caches. A command issued by a device gets broadcast to all other system devices simultaneously and in the same clock cycle that the command is placed on the bus. A bus enforces a fixed ordering on all commands placed on it. This order is agreed upon by all devices in the system since they all observe the same commands. The devices can also agree, without special effort, on the final effect of a sequence of commands. This is a major advantage for a single-bus-based multiprocessor.
A single-common-bus design, however, limits the size of the system unless one opts for lower system performance. The limits of technology typically allow only a few devices to be connected on the bus without compromising the speed at which the bus switches and, therefore, the speed at which the system runs. If more master devices, such as processors and I/O agents, are placed on the bus, the bus must switch at slower speeds, which lowers its available bandwidth. Lower bandwidth may increase queuing delays, which result in lowering the processor utilization and system performance.
Another serious shortcoming in a single-bus system is the availability of a single data path for transfer of data. This further aggravates queuing delays and contributes to lowering of system performance.
Two broad classes of cache-coherence protocols exist. One is bus-based snooping protocols, wherein all the caches in the system connect to a common bus and snoop on transactions issued on the common bus by other caches and then take appropriate actions to stay mutually coherent. The other class is directory-based protocols, wherein each memory address has a xe2x80x9chomexe2x80x9d site. Whenever a cache accesses that address, a xe2x80x9cdirectoryxe2x80x9d at the home site is updated to store the cache""s identity and the state of the data in it. When it is necessary to update the state of the data in that cache, the home site explicitly sends a message to the cache asking it to take appropriate action.
In terms of implementation and verification complexity, the bus-based snooping protocol is significantly simpler than the directory-based protocol and is the protocol of choice of symmetric multiprocessor (SMP) systems. However, the bus-based snooping protocol is effectively employed in a system with only a small number of processors, usually 2 to 4. Thus, although a single-system-bus design is the current design choice of preference for implementing coherence protocol, it cannot be employed for a large-way SMP with many processors.
However, in a large-way, distributed, multi-bus, multiprocessor, system using a bus-based snooping protocol, it is possible for two-related transactions to collide with one another due to shared resources. In certain circumstance, the transaction collision can result in a livelock where both transactions fail to make forward progress because of destructive interference between the two transactions. A livelock condition may be easily broken through the use of dedicated signals between components experiencing a livelock.
Once a decision is made to design a large-way, distributed multiprocessor system with multiple buses, there are several design challenges for ensuring efficient data transfers. The number of connections to centralized control units can become substantial. Pin count on the physical components becomes a significant limitation, especially in a system that supports a large address space with large data transfers. Hence, it is generally desirable to limit the number of signals so as to limit the number of physically separate pins.
Therefore, it would be advantageous to have a large-way SMP design using bus-based cache-coherence protocols with the ability to break livelocks without additional dedicated sideband signals.
A distributed system structure for a large-way, symmetric multiprocessor system using a bus-based cache-coherence protocol is provided. The distributed system structure contains an address switch, multiple memory subsystems, and multiple master devices, either processors, I/O agents, or coherent memory adapters, organized into a set of nodes supported by node controllers. A node controller receives transactions from a master device, communicates with a master device as another master device or as a slave device, and queues transactions received from a master device. Since the achievement of coherency is distributed in time and space, a node controller helps to maintain cache coherency. The node controllers must give simultaneous address bus grants to the address switch to initiate a snoop. Livelocks are detected individually by each node controller in an uncoordinated manner from a lack of successful snoops from the address switch to the node controllers. To break the livelock, address bus grants to the address switch are manipulated by the node controllers in a distributed, uncoordinated manner until a snoop is seen by the node controllers, thereby obviating the need for dedicated sideband signals on the bus.